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The Power of Sound

Sound waves in "thermoacoustic" engines and refrigerators can replace the pistons and cranks that are typically built into such machinery

Steven Garrett, Scott Backhaus

An Acoustic Laser

The transparency of this device, literal and figurative, invites analogies with the laser. Borrowing some vocabulary from optics, one would say that a non-equilibrium condition (corresponding to the population inversion of electron energy levels in a laser material) is maintained across the heated stack. The test tube amounts to an acoustic resonator, which, like a laser cavity, allows a standing wave to build in amplitude as energy bounces back and forth. The open side of the test tube serves the same function as the partially silvered mirror at the output side of a laser. Both allow some of the energy stored within the resonant cavity to radiate into the surrounding environment. Although Chen's "acoustic laser" produces only about a watt of sound power, a similar device heated by the burning of natural gas produces in excess of 10 kilowatts—a high-powered laser indeed!

Figure 4. Thermoacoustic enginesClick to Enlarge Image

One of the most remarkable features of such thermoacoustic engines is that they have no moving parts. They demand nothing beyond the basic physics of the cavity and stack to force the compressions, expansions, displacements and heat transfers to happen at the right times. The internal-combustion engines in our cars also depend on proper timing—the intake, compression, expansion and exhaust stages of the power cycle must take place in smooth succession. But conventional automobile engines require at least two valves per cylinder, each with a spring, rocker arm and a push rod (or an overhead cam driven by a timing belt) to produce the required phasing. This difference makes thermoacoustic devices much simpler and potentially much more reliable than conventional engines and refrigerators, because they can avoid wear associated with valves, piston rings, crankshafts, connecting rods and so forth. Thus thermoacoustic devices require no lubrication.

To the uninitiated, it may seem surprising that pistonless engines can achieve high power levels. Thermoacoustic devices manage this feat by exploiting acoustic resonance to produce large pressure oscillations from small gas motions. Consider a closed tube (an acoustic resonator) with a loudspeaker mounted at one end. The oscillating movement of the loudspeaker pumps in acoustic energy, which travels down the tube at the speed of sound, reflects off the far end and shoots back toward the source. If the frequency of the excitation is just right, the next increment of energy that the loudspeaker injects will arrive in step with the reflected portion of the acoustic wave.

The pressure swings in the resonating wave will then grow until the energy added during one cycle is exactly equal to the energy dissipated during one cycle, either by friction or by the production of useful work. The ultimate value of the pressure variation depends on the quality factor of the resonator, Q (which is equal to 2/π times the ratio of the pressure the loudspeaker produces in the resonator to that which the same loudspeaker would have generated in an infinitely long tube, one in which there would be no reflected wave).

The result of this resonant Q amplification can be easily understood by considering the motion of a piston compressing some gas within a cylinder. If the initial length of the gas volume is, say, 20 centimeters and the piston moves slowly inward 1 centimeter, the pressure of the gas would increase by 5 percent, assuming no leakage around the piston. If, however, it oscillated back and forth rapidly at the resonant frequency of the cavity (860 cycles per second, assuming that the cylinder is filled with air at room temperature so that exactly one-half wavelength of sound fits inside), the piston would only have to move by something like 0.05 millimeter in a typical cavity (Q=30) to produce the same change in pressure. That tiny distance is only one two-hundredth as far as in the case of slow compression, yet it achieves exactly the same peak pressure.

Clearly, an oscillating acoustic source that moves such small distances does not need a piston with sealing rings moving in a lubricated cylinder—eliminating all sorts of pesky components found in conventional refrigeration compressors and internal-combustion engines. Flexible seals, such as metal bellows, would suffice. Such seals require no lubrication and do not demand the machining of close-tolerance parts to eliminate gas "blow-by" between a piston and its tight-fitting cylinder.

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